The present invention relates to a hot rolled plate of low carbon martensite stainless steel having excellent formability and corrosion resistance, which is suitable to be used as material for welded pipes such as line pipes, oil casing and tubing goods or pipes for petrochemical facilities, as well as a manufacturing process of the same, and a welded pipe made thereof.
A low carbon martensite stainless steel has been recently developed as materials for an oil well. Such a low carbon martensite stainless steel is relatively inexpensive as it has a less content of expensive elements such as chromium than a duplex stainless steel, and moreover shows an excellent corrosion resistance when it is used in a wet environment containing carbonic dioxide or mixture of carbonic dioxide and a very small amount of hydrogen sulfide gas. Since the martensite stainless steel is low in carbon contents, it has an excellent weldability, and thus suitable for a line pipe assuming a circumferential welding by gas tungsten arc welding (referred to as GTAW hereinafter) or gas metal arc welding (referred to as GMAW hereinafter).
The steel pipes made of a low carbon martensite stainless steel have been conventionally manufactured mainly for a seamless pipe. However, a demand for seamless pipes of 10 mm or less in thickness, which are difficult to manufacture, has been increased in recent years.
Actually there is very few instances where a welded pipe which made of a low carbon martensite stainless steel has been put in practical use. In Japanese Patent Application Laid-Open (JP-A) Nos. 4-191319 and 4-191320, however, a welding process has been proposed in which a material coil is formed into a pipe shape and the butt portions thereof, are welded by electric resistance welding (referred to as ERW hereinafter). Additionally, in a small diameter pipe, butt welding effected by GTAW or plasma arc welding (referred to as PAW hereinafter) has also been studied.
Furthermore, as a new welding method which has been developed recently there is a laser welding-pipe making method. As examples where this method is applied for manufacturing a small diameter pipe, there are Japanese Patent Application Laid-Open (JP-A) Nos. 63-278688 in which an austenite stainless steel is used as a material steel, Japanese Patent Application Laid-Open (JP-A) Nos. 63-278689 in which a ferrite stainless steel is used as a material steel, and Japanese Patent Application Laid-Open (JP-A) Nos. 63-278690 in which an alloy containing molybdenum is used as a material steel. In these examples, it has been proposed that, after a pipe is manufactured by laser welding, a welded seam portion thereof is subject to a post weld heat treatment, so that a mechanical property of the welded metal is restored and excellent performance thereof is achieved.
In addition, butt welding using a laser oscillator with a larger power has been developed recently. In Japanese Patent Application Laid-Open (JP-A) Nos. 9-164425, a process has been proposed in which a pipe is manufactured by butt laser welding, and then the manufactured pipe is subject to an adequate post weld heat treatment at its portion near the welded seam portion so that excellent corrosion resistance can be obtained.
In recent years, a poor formability when a hot rolled plate of a low carbon martensite stainless steel is used as a material steel to be formed and welded in a pipe shape has been apparent as a significant problem. When a thin steel plate having a high strength is formed in a pipe shape, an edge wave defect estimated as a buckling phenomenon due to compression stress acting in a longitudinal direction of the pipe occurs at butted end faces, thereby resulting in a poor butt welding. Also, a thick steel plate of not less than 10 mm in thickness is used, forming or production itself becomes very difficult, and a mechanical load to a production facility such as a production roll is increased.
This phenomenon has been considered to occur mainly because a steel plate is strengthen excessively due to solution hardening of alloy elements such as nickel or molybdenum in martensite structure and due to residual strain in a hot rolled coil. In particular, a steel plate which is hot rolled often has a yield stress (YS) of higher than 110 ksi (758 Mpa), thereby making it very hard to be softened even when it is annealed or tempered only in an ordinary manner, unlike a low alloy steel. In the present situation, welding is performed without any established solution to this problem.
Currently, a strength required for a line pipe is mainly 80 ksi class which is in a range of 80 to 95 ksi (551 to 654 Mpa) in yield stress (YS), and the line pipe need not have an unnecessarily high strength. If the strength of a line pipe is excessively high, corrosion resistance such as sulfide stress cracking (referred to as SSC hereinafter) in wet environment containing hydrogen sulfide or mechanical properties such as toughness deteriorate in many cases.
An object of the present invention is to provide a hot rolled plate of low carbon martensite stainless steel which is suppressed from being excessively strengthened, and still is excellent in formability and corrosion resistance suitable as a material for a welded pipe, as well as a manufacturing process of the same, and a welded pipe made thereof.